The present invention relates to automatic retransmission protocols, and more particularly to improved link layer protocol synchronization associated with automatic retransmission protocols.
The purpose of a wireless communication system is to successfully transmit information from a transmitter to a receiver over a communication channel. In wireless communication systems, bit errors occur during transmission due to noise and multi-path fading. A variety of error control techniques are available for combating transmission errors and reducing bit errors. The hybrid automatic repeat request (HARQ) protocol, which combines ARQ with forward error correction (FEC), represents one exemplary error control technique. ARQ adds redundant bits or check bits to a protocol data unit (PDU) to enable detection of errors at the receiver. If the receiver detects errors in the received PDU, the receiver may send a feedback message, (e.g., a NACK) on a control channel that request a repeat transmission of the PDU. FEC uses error-correcting codes to combat errors by adding redundancy to the PDU before it is transmitted. The added redundancy enables the receiver to detect and correct most errors that occur during transmission.
While HARQ provides robustness against link adaptation errors for high-speed downlink packet access (HSDPA) channels and enhanced uplink channels, problems may occur when the feedback message is erroneously interpreted by the transmitter. Such interpretation errors may lead to residual errors after HARQ operations. Further, such interpretation errors may lead to the loss of link layer protocol synchronization between the transmitter and receiver. For example, if the transmitter misinterprets a NACK associated with a PDU as an ACK, the transmitter will transmit a new protocol data unit (PDU) instead of retransmitting the previous PDU. Because the receiver does not receive the expected retransmission, the receiver loses link layer protocol synchronization with the transmitter. Further, combining errors may occur at the receiver when the receiver soft combines different PDUs, and attempts to decode the soft combination. It is therefore desirable to reduce the effects of feedback message errors.